Sinuous wire springs have a long history of being used in furniture items, such as various seating units. They are known in the industry to serve various functions with a relatively minimum amount of wire, which can translate into lower costs. A sinuous wire spring is made up of a series of parallel “bars” connected by a series of semi-circular “loops” to form a serpentine-like configuration. Often a pre-determined number of bars and loops are used to create a sinuous spring of a specific length. The two ends of the sinuous spring are typically terminated in a “safety end”, to keep the spring from slipping out of its rail attachment clip, or in a specialty end designed for attachment to metal rails, for instance.
Sinuous springs might be fabricated using various techniques. For example, the loops might be bent, stamped, or formed into the wire. Commonly, once the wire is formed into a sinuous configuration, the entire spring is further formed into a circular shape of a given radius, and put through a stress relief process. Many options of gauge of wire and finished length of the spring are available to suit various applications.
Sinuous springs are typically installed in a wood or metal frame by pulling the spring between the front and back rails of the frame, creating tension on the spring due to the un-coiling of the circular shape, and some stretching of the overall spring. The performance of the sinuous spring is primarily due to the torsion action of the flat bars of the spring. Sinuous springs are typically unitized in the seat frame by connecting them together by means such as metal hooks, or by clipping one or more pieces of paper or plastic covered “stake wire” to the flat bars of the sinuous, across the width of the seat, so that the individual sinuous will be tied together to act in a more unified fashion.
In some arced springs, when the spring is pulled between the rails, the arc of the spring is never pulled completely flat, creating a crown to the sinuous spring that adds to the ride and liveliness of the seat, as well as to the durability. But the radius of the spring can be increased in the stress relief process, in which case the crown of the seat is reduced. When the spring is manufactured flat, or very nearly flat, then the crown of the seat is eliminated. Reducing the radius or providing a flat spring can reduce the performance of the seat, but are sometimes chosen because the springs can be easier to handle and install.